Added Glimpse tab for Glimpse internals
Reworked lifetime management of the GlimpseRequestContextHandle

AspNetRequestResponseAdapter: Removed dependency on HttpContext.Items
collection and moved the logic of checking whether or not HTTP Headers
have been sent into the class

GlimpseRuntime : Reworked runtime to allow the framework providers to
manage the lifetime of the GlimpeRequestContextHandles

HttpModule : Reworked to deal with handling the lifetime of the
GlimpseRequestContextHandle

GlimpseMiddleware : Reworked to deal with handling the lifetime of the
GlimpseRequestContextHandle

HttpContextExtensions : class has been removed since that the checking
for HTTP headers has been put inside the AspNetRequestResponseAdapter
and the same for the GenerateGlimpseScriptTags, all of this exposed on
the IAspNetRequestResponseAdapter interface.
Reimplemented GlimpseConfiguration.TimerStrategy